In recent years, from a point of view of global environmental protection, a reduction in weight of a vehicle body of automobile has been required as a part of reduction in CO2 emissions from automobiles, and a high-strengthening of a steel material for automobile has been aimed. This is because, by improving the strength of steel material, it becomes possible to reduce a thickness of the steel material for automobile. Meanwhile, a social need with respect to an improvement of collision safety of automobile has been further increased, and not only the high-strengthening of steel material but also a development of steel material excellent in impact resistance when a collision occurs during traveling, has been desired.
Here, respective portions of a steel material for automobile at a time of collision are deformed at a high strain rate of several tens (s−1) or more, so that a high-strength steel material excellent in dynamic strength property is required.
As such a high-strength steel material, a low-alloy TRIP steel having a large static-dynamic difference (difference between static strength and dynamic strength), and a high-strength multi-phase structure steel material such as a multi-phase structure steel having a second phase mainly formed of martensite, are known.
Regarding the low-alloy TRIP steel, for example, Patent Document 1 discloses a strain-induced transformation type high-strength steel sheet (TRIP steel sheet) for absorbing collision energy of automobile excellent in dynamic deformation property.
Further, regarding the multi-phase structure steel sheet having the second phase mainly formed of martensite, inventions as will be described below are disclosed.
Patent Document 2 discloses a high-strength steel sheet having an excellent balance of strength and ductility and having a static-dynamic difference of 170 MPa or more, the high-strength steel sheet being formed of fine ferrite grains, in which an average grain diameter ds of nanocrystal grains each having a crystal grain diameter of 1.2 μm or less and an average crystal grain diameter dL of microcrystal grains each having a crystal grain diameter of greater than 1.2 μm satisfy a relation of dL/ds≥3.
Patent Document 3 discloses a steel sheet formed of a dual-phase structure of martensite whose average grain diameter is 3 μm or less and martensite whose average grain diameter is 5 μm or less, and having a high static-dynamic ratio.
Patent Document 4 discloses a cold-rolled steel sheet excellent in impact absorption property containing 75% or more of ferrite phase in which an average grain diameter is 3.5 μm or less, and a balance composed of tempered martensite.
Patent Document 5 discloses a cold-rolled steel sheet in which a prestrain is applied to produce a dual-phase structure formed of ferrite and martensite, and a static-dynamic difference at a strain rate of 5×102 to 5×103/s satisfies 60 MPa or more.
Further, Patent Document 6 discloses a high-strength hot-rolled steel sheet excellent in impact resistance property formed only of hard phase such as bainite of 85% or more and martensite.